Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis (or steam cracking) furnace that has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products, including olefins, leave the pyrolysis furnace for further downstream processing.
Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules. The pyrolysis process, however, produces molecules that tend to combine to form high molecular weight materials known as tar. Tar is a high-boiling point, viscous, reactive material that can foul equipment under certain conditions. In general, feedstocks containing higher boiling materials tend to produce greater quantities of tar.
Conventional steam cracking systems have been effective for cracking high-quality feedstock which contains a large fraction of light volatile hydrocarbons, such as ethane, and naphtha. However, steam cracking economics sometimes favor cracking lower cost heavy feedstocks such as, by way of non-limiting examples, gas oil, crude oil and atmospheric residue. Gas oil, crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points in excess of about 590° C. (1100° F.) otherwise known as resids.
Cracking heavier feeds, such as residues, kerosenes and gas oils, produces large amounts of tar, which typically contains high-boiling and/or non-volatile components including paraffin-insoluble compounds, such as pentane-insoluble (PI) compounds or heptane-insoluble (HI) compounds, which are molecules of high molecular weight with multi-ring structures, e.g., asphaltenes. These materials reduce the economic value of tar by rendering it highly viscous and less compatible for mixing with highly paraffinic streams, inducing precipitation of the paraffin-insoluble components from the resulting mixture.
Various methods are known in the art to treat tars produced from steam cracking.
U.S. Pat. No. 3,691,058, incorporated herein by reference in its entirety, discloses depolymerization and subsequent hydrocracking to break down steam cracked tars from gas oils containing condensed ring aromatics into single-ring aromatics.
U.S. Pat. No. 3,310,484, incorporated herein by reference in its entirety, discloses thermal depolymerization in methylnaphthalene of asphaltenes obtained from a crude oil.
U.S. Pat. No. 3,384,448, incorporated herein by reference in its entirety, teaches thermal depolymerization of a crude oil and vanadium recovery therefrom.
U.S. Pat. No. 4,310,409, incorporated herein by reference in its entirety, discloses hydrogenating distillates and deasphalted fractions, e.g., gas oil, vacuum gas oil, deasphalted atmospheric, vacuum residue, visbreaker or coker distillates. The heavy hydrogenated fraction is subjected to thermal cracking.
U.S. Pat. No. 4,257,871, incorporated herein by reference in its entirety, teaches preparation of olefins from deasphalted vacuum residue by blending the asphalt-depleted product with a lighter fraction, e.g., vacuum gas oil, and hydrogenating the blend, followed by thermal cracking.
U.S. Pat. No. 6,149,800, incorporated herein by reference in its entirety, teaches preparation of olefins by hydroprocessing a feed such as deasphalted oil using a countercurrent hydrogen-containing treatment, followed by thermal cracking in a steam cracker.
U.S. Pat. No. 6,190,533, incorporated herein by reference in its entirety, discloses converting hydrocarbons such as visbreaker oil or deasphalted oil into steam cracked products by hydrotreating to remove organic sulfur and/or nitrogen compounds, and then passing to a steam cracking zone.
U.S. Pat. No. 6,210,561, incorporated herein by reference in its entirety, discloses steam cracking a visbreaker oil or deasphalted oil which has been hydrotreated with aromatics saturation.
U.S. Pat. No. 6,303,842, incorporated herein by reference in its entirety, discloses the production of olefins by thermally steam cracking residua feedstocks. Feedstock such as a petroleum residuum can be hydrotreated, if necessary, and subjected to deasphalting prior to hydrotreatment, if required.
Hydrocarbon Processing, 65(11), pp. 84-86, November, 1986, discloses hydrocracking low grade vacuum-flashed distillates to provide hydrogenated residue (hydrowax) to provide a feed for an ethylene plant.
U.S. application Ser. No. 12/023,204, filed Jan. 31, 2008, incorporated herein by reference in its entirety, discloses upgrading steam cracker tar by heating from below 300° C. to a temperature above 300° C. for a time sufficient to convert at least a portion of the steam cracked tar to lower boiling molecules.
U.S. application Ser. No. 12/099,971, filed Apr. 9, 2008, incorporated herein by reference in its entirety, discloses upgrading steam cracker tar by heating from below 300° C. to a temperature above 300° C. in the presence of steam and for a time sufficient to convert at least a portion of the steam cracked tar to lower boiling molecules.
It would be desirable to provide an apparatus and process to convert steam cracker tar to more valuable, lower boiling materials, which can be used as a steam cracker feed, while minimizing the production of unwanted steam cracked by-products. Moreover, it would be especially desirable to provide a steam cracker feed derived from steam cracker tar which is substantially reduced in tar asphaltenes or other polymers that can undergo high conversion catalytic hydrogenating (greater than about 5 wt. % conversion) while minimizing fouling of the hydrogenating catalyst.